1. Field of the Invention
The present invention relates to earth-boring bits of the rolling cutter variety. Specifically, the present invention relates to improving the wear resistance of earth-boring bits.
2. Background Information
The success of rotary drilling enabled the discovery of deep oil and gas reserves. The rotary rock bit was an important invention that made that success possible. Only soft formations could be commercially penetrated but with the earlier drag bit. The original rolling-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 939,759, drilled the hard caprock at the Spindletop field, near Beaumont Texas, with relative ease.
That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of modern rotary rock bits. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today often drill for miles. Many individual improvements have contributed to the impressive overall improvement in the performance of rock bits.
Earth-boring bits typically are secured to a drill string, which is rotated from the surface. Drilling fluid or mud is pumped down the hollow drill string and out of the bit. The drilling mud cools and lubricates the bit as it rotates and carries cuttings generated by the bit to the surface.
Rolling-cone earth-boring bits generally employ cutting elements on the cutters to induce high contact stresses in the formation being drilled as the cutters roll over the bottom of the borehole during drilling operation. These stresses cause the rock to fail, resulting in disintegration and penetration of the formation material being drilled.
Operating in the harsh down hole environment, the components of earth-boring bits are subjected to many forms of wear. Among the most common forms of wear is abrasive wear caused by contact with abrasive rock formation materials. Moreover, the drilling mud, laden with rock chips or cuttings, is a very effective abrasive slurry.
Many wear-resistant treatments are applied to the various components of the rock bit. Among the most prevalent is the application of a welded-on wear-resistant material or xe2x80x9chardfacing.xe2x80x9d This material can be applied to many surfaces of the rock bit, including the cutting elements. Commonly assigned U.S. Pat. No. 3,158,214 to Wisler et al., discloses application of hardfacing to the xe2x80x9cshirttailxe2x80x9d of the bit, a portion of the bit body immediately above the cutters that contacts the sidewall of the borehole and is subject to great abrasive wear. Another solution applied to the shirttail region is a plurality of wear-resistant inserts, similar to those used in the cutters, secured by interference fit in the shirttail. Sometimes, these inserts are designed to cut or actively engage the sidewall of the borehole to act as stabilizers in addition to wear pads.
A need exists, therefore, for earth-boring bits of the rolling-cutter variety having improved means for resisting abrasive wear.
It is a general object of the present invention to provide an earth-boring bit of the rolling cutter variety having an improved wear-resistant treatment of the shirttail.
This and other objects of the present invention are accomplished by providing an earth-boring bit having a bit body that is threaded at one end for connection into a drill string. At least one cantilevered bearing shaft depends inwardly and downwardly from the bit body. A cutter is mounted for rotation on each bearing shaft and includes a plurality of cutting elements arranged in generally circumferential rows. At least one wear-resistant element secured to the bit body between the lower end of the bit body and the threaded end, the wear-resistant element being elongate or generally oblong in cross-section.
According to the preferred embodiment of the present invention, the bit body comprises at least one bit leg from which the bearing shaft depends. A shoulder is defined on each bit leg below the threaded end of the bit body and a shirttail portion on each bit leg proximal the bearing shaft. The oblong wear-resistant element is secured to the shirttail portion of the bit body between the shirttail and the shoulder.
According to the preferred embodiment of the present invention, the wear-resistant element is formed of hard metal and is secured by interference fit in an aperture in the bit body.
According to the preferred embodiment of the present invention, a plurality of the wear-resistant elements are secured to the bit body between the shoulder and the shirttail, the majority of the wear-resistant elements being secured to the bit body proximal a leading edge of each bit leg.
According to the preferred embodiment of the present invention, the bit has a gage diameter and the wear-resistant elements project from the bit body an amount less than the gage diameter of the bit.